A standard left ventriculomyotomy and myectomy (LVMM) has been performed for relief of left ventricular outflow tract obstruction secondary to idiopathic hypertrophic subaortic stenosis (IHSS) in 463 patients. This report summarizes 133 patients undergoing an LVM&M since January 1982. An attempt has been made to define criteria for choice of operation, LVM&M vs. mitral valve replacement (MVR) based upon septal thickness, distribution of hypertrophy, level of artery disease. Intraoperative 2-D and M-mode echos have been performed on a number of these patients providing precise data utilized intraoperatively. Patients with concomitant CAD are at greater risk for an iatrogenic VSD creation which may be avoided by a modified LVM&M or MVR. Operative mortality is 3.0% and late mortality 3.0% Results are presented based on preoperative resting gradients less than 50 mm Hg and greater than 50 mm Hg. Postoperative hemodynamic studies reveal good relief of resting gradient in most patients but significant provokable gradients remain in some patients. Three patients have demonstrated significant RVOT obstruction (greater than 50 mm Hg) and underwent concomitant LVM&M and resection of RVOT obstruction. Two patients developed late VSD's; one required closure for a QP:QS of 2.6:1. Reoperation has been performed in some patients with persistent symptoms and gradients. Medical therapy is continued in patients with significant gradients regardless of symptomatic status.